In electronic displays, transistors in a thin film form (thin film transistors, TFTs) are required to form a two dimensional arrays for switching of light emitting devices or light valves. For electronic display applications, it is desirable to have the TFTs with a low series resistance in the ON state and small leakage current in the OFF state in order to reduce the unwanted joule heating loss and hence to reduce power consumption in operation. The series resistance of a TFT in ON state is mainly determined by the transport property (electron mobility and charge carrier density) of the channel layer semiconductor materials and it is inversely proportional to the charge carrier mobility and the sheet charge carrier density (the number of charge carriers per unit area). For TFTs with normal structures, the capacitances between the control gate and the channel layer and the gate operation voltage for ON state are limited. Hence, the sheet charge carrier density of the channel layer in ON state has a limited value: in the order of 1014 charge carriers per cm2 or less. Therefore, the unwanted series resistance in ON state is mainly determined by the charge carrier mobility.
Until very recently in the electronic display technology, a majority of displays employs TFTs backplane with amorphous Si layer as the channels which has an electron mobility of 2 cm2/V-sec or less. A small fraction of displays employ TFTs with polycrystalline Si which has an electron mobility of about 100 cm2/V-sec, but these require more expensive equipment and substrates for preparation. More recently, TFTs with metal oxide or metal oxynitride semiconductors as the channel layers have been developed. These metal oxide and metal oxynitride TFTs have the benefits of low deposition temperature and high mobility, which is significantly higher than that for the amorphous Si TFTs but often still less than that for the polycrystalline Si TFTs. It is thus useful to develop thin film transistors with charge carrier mobility greater than 100 cm2/V-sec and without the need of extensive deposition equipment and processes.
In radio frequency (RF) applications, transistors are used either as amplifiers or as switches and they are operated at high frequencies and must have high switching speed. In order to have high switching speed, transistors require to have high charge carrier mobility and small parasitic capacitances. In most of the Si technology, the field effect mobility of electrons is 350 cm2/V-sec and the hole mobility is 90 cm2/V-sec. In compound semiconductor technology, the field effect electron mobility is as high as 1500 to 6000 cm2/V-sec for monocrystalline GaN and GaAs. However, both Si and compound semiconductor technologies required extensive equipment and processes. It would be important to develop device technology to provide transistors which can be deposited at low temperatures and with a high mobility using less extensive equipment and processes for RF applications.